Yarns each having stretchability have been adopted as yarns for use in leg knitted fabric products such as stockings. Polyurethane elastic fibers obtained by dissolving a general-purpose polyurethane in a solvent and spinning the solution have been known as such yarns each having stretchability. However, the polyurethane elastic fibers are not used alone in products, and are generally used as a mixture with long fibers or various short fibers made of, for example, nylon and polyester. For example, composite yarns obtained by covering the polyurethane elastic fibers, such as filament-twisted yarns (FTY's) including a single-covered yarn (SCY) and a double-covered yarn (DCY) have been known. However, each of the FTY's has such a drawback as to be poor in transparency because of its bulkiness due to the covering of a core yarn with a front yarn.
Conjugated yarns have been known as yarns each having stretchability and superior in transparency to an FTY (Patent Document 1), and leg knitted fabric products produced from knitted fabrics using the conjugated yarns have been hitting the market. The conjugated yarns are composite yarns spun by using various kinds of resin components. The leg knitted fabric products produced from the knitted fabrics using the conjugated yarns are superior in transparency to leg knitted fabric products produced from knitted fabrics using covering yarns.
However, the conventional conjugated yarns are generally poor in strength and stretchability. In view of the foregoing, each of the conventional conjugated yarns has expressed its strength and stretchability by stretching a complete yarn under heat to provide a crimp.
However, the tenacity of a gray yarn itself reduces owing to heat setting, though an apparent strength increases by virtue of a reduction in gray yarn fineness resulting from the stretching. In addition, the tactile senses of leg knitted fabric products produced from knitted fabrics using yarns provided with crimps are impaired by the presence of the crimps.
Conjugated yarns each using a cross-linked resin obtained by adding polyisocyanate to a general-purpose polyurethane in its core portion have been reported as conjugated yarns each having excellent stretching elasticity (Patent Document 2). The polyurethane obtained by the method has good stretchability but has an insufficient power feeling, and hence involves the following problems when turned into a knitted fabric. The knitted fabric is poor in wearing feeling and durability.
Conjugated yarns spun by using a general-purpose polyurethane and a polyester-based elastomer have been reported as recent conjugated yarns (Patent Document 3). The melting viscosity of each of the general-purpose polyurethane and the polyester-based elastomer must be appropriately adjusted upon conjugation. In particular, when the general-purpose polyurethane is made comparable in melting viscosity to the polyester-based elastomer, part of the polyurethane thermally decomposes, and hence problems in terms of spinnability and the physical properties (e.g., stretchability) of the resultant yarns, such as a discharge failure and a reduction in degree of symmetry, may arise.